An automatic blood pressure measuring apparatus is known that includes a compression band wrapped around a compressed site of a living body and that sequentially extracts a pulse wave that is pressure oscillation in the compression band in the process of changing a compression pressure value of the compression band to determine a blood pressure value of the living body based on a change in the pulse wave. Since an expansion bag included in the compression band needs a sufficiently large compression width dimension relative to a diameter of a compressed site and has a relatively large capacity, the pulse wave, i.e., the pressure oscillation, generated in response to a change in capacity of an artery in the compressed site tends to be a weak signal and therefore causes reduction in measurement accuracy.
In this regard, as described in Patent Document 1, to clearly detect a change in capacity of an artery, a two-layer structure compression band is proposed and the compression band is disposed with a detection expansion bag having a capacity smaller than a main expansion bag such that the entire bag overlaps in the width direction with a portion of inside of the main expansion bag, and has a shield plate disposed between the detection expansion bag and the main expansion bag. According to this compression band, while a portion of the main expansion bag directly pressurizes a compressed site, another portion of the main expansion bag pressurizes the compressed site indirectly through the detection expansion bag.